1. Field of the Invention
The present invention relates to a multi-layered holographic read-only memory, preferably used as a mobile (or portable) memory card such as a magnetic card and an IC card. This type of card is difficult to forge or reproduce, and thus can be used as an authentication card for electronic commercial transaction. Additionally, the present memory has a large capacity and can be manufactured at low cost, and thus is suitable for distributing software for music, pictures, computer applications, or the like.
This application is based on Patent Applications Nos. Hei 10-32578, Hei 10-44941, and Hei 10-75336 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Magnetic cards such as a telephone card are conventionally used as mobile (or portable) information cards which a user can carry in a pocket. Recently, IC cards have become practical to use, and application of the IC cards to the electronic commercial transaction has been investigated. The magnetic cards are cheap, but may be forged. The IC cards are difficult to forge, but their cost per bit is expensive.
The holographic storage technique is another technique applicable to a data memory which is difficult to forge and has a large capacity. Holography can be classified into thin film holography and volume holography.
A volume holographic memory has a larger storage capacity; however, no data duplication technique suitable for the volume hologram exists. Therefore, this type does not suit mass production, and it is difficult to apply the volume holography to a read-only memory used as an authentication card or used for distributing software applications.
A thin film hologram can be mass-produced using a printing technique, but has a limited memory density. Therefore, in consideration of the size and convenience of a necessary data retrieval device, the memory using thin film holography has less appeal in comparison with the IC card. Even if such thin film holograms are multi-layered so as to solve the above problem and to increase the storage capacity, holographic images, reconstructed from each hologram by using an ordinary reconstruction method, are simultaneously reconstructed. Therefore, due to the crosstalk being large, necessary data or information cannot be obtained.